In U.S. Pat. Nos. 4,219,928 and 4,249,304, and corresponding British Pat. Nos. 2,052,134 published on Apr. 20, 1983 and 2,050,208 published on July 20, 1983, an electrical cable assembly and system are shown for installation beneath an overlying carpeting material in offices and like areas.
The cable assembly includes an elongate flat cable having flat electrical conductors encased in electrical insulation, with a protective overlayer, in the form of a metallic shield displaceable from the cable for connection purposes, and a protective underlayer to permit laying of the cable assembly directly upon unfinished surfaces, such as concrete floors.
Typically, for single phase installation, the primary or feed cable has three conductors, termed line, neutral or return and grounding conductors. This cable is connected to power mains and extends radially, i.e., lengthwise, into the office. Secondary flat multiconductor cables are placed in overlying relation to the feed cable and led therefrom transversely to locations at which outlets are desired for powering of lights, typewriters, computer terminals, etc. At its end distal from the feed cable, each secondary cable is connected to a wall outlet or the like.
At the location of registry of the secondary and primary cables, individual conductors of the secondary cable are connected, as by insulation-piercing connectors, to corresponding conductors of the primary cable. Such "tap" connections are made with the shields displaced from the cables and the connections are suitably electrically insulated from the ground shield prior to replacement of the shields in overlying disposition to the connected cables. Each shield is secured to its cable by electrical and mechanical connection to the cable grounding conductor, e.g., the shield is welded to the grounding conductor at locations spaced along the length of the cable. This feature not only permits one to cut the shield for tap or splice connections to the cable at virtually any location, but insures that such interruption of integrity of the shield by cutting will not give rise to a discontinuity in the protective electrical connection of the shields to electrical ground.
While deployment of the foregoing wiring system as a direct substitute for wiring systems of traditional character disposed in costly raceways, ductwork and the like in the United States was in initial design requirement, implementation of the system in ring-main practice, such as in Great Britain, was not an original design parameter. A basic disparity, existing as between United States and British wiring practices, precludes direct use in Great Britain of the three-conductor system above described. Thus, in British ring-main practice commencing in the early nineteen fifties, one forms a loop fully about the room being wired, three conductors (line, neutral and earth) encircling the room. The opposite ends of the individual conductors are connected to one another at a feed location and then connected to the power mains. The conductors are led within so-called skirting at baseboard level and over plinths into architraves and therethrough in traversing doorways. Connections are made in parallel circuit from the baseboard skirting to wall receptacles through secondary skirting. Receptacles are distributed spacedly circumferentially of the loop and, under British practice, may be introduced at will since plugs are individually electrically fused in accordance with the load they present to the system.
Although the British baseboard and architrave skirting arrangements track the traditional American ducting and raceway needs as respects complexity of wiring system installations heretofore available and difficulty of later modification, British technology in this area would not appear to be enhanced by the introduction of undercarpet wiring in the form above discussed. While one concievably could take the three-conductor flat cable and loop the same about the room, the comparative installation cost would not be as much reduced by such practice versus ring-main British practice as is the undercarpet wiring system installation cost reduced versus traditional practice in the United States. In this possible British approach, installation would be cost-burdened by the cable assembly material required to add three legs, namely, two cross legs and the return leg of cable and shield, to transform the single feed cable running radially in one direction into a full loop.